Switching roles from antagonist to mutualist: a harvester ant as a key seed disperser of a myrmecochorous plant

1. Harvester ants are major seed predators in arid environments. However, given that many harvester ants are partly omnivorous and therefore potentially attracted to the elaiosomes of myrmecochorous seeds, it is unclear if these ants act as predators or dispersers when removing myrmecochorous seeds. 2. We describe the outcomes of interactions between the harvester ant, Pogonomyrmex naegelii, and myrmecochorous plant, Microstachys serrulata, in a Brazilian savanna. We: (i) evaluated the role of elaiosome in seed removal by P. naegelii; (ii) investigated the fate and viability of removed seeds; (iii) tested if soils associated with P. naegelii nests are nutrient-enriched; (iv) compared seedling survival; and (v) the density of seedling and adult M. serrulata nearby to P. naegelii nests compared with those away from these nests (i.e. controls). 3. Rates of removal of M. serrulata seeds were two-fold higher with elaiosomes than without. The ant attractant oleic acid was the dominant fatty acid in elaiosomes, but it was absent from seeds. Removed seeds are taken into nests, and Tetrazolium tests indicated that 95% of seeds remain viable. Soils associated with P. naegelii nests were not nutrient-enriched, and seedling survival was similar nearby to P. naegelii nests compared with control areas. However, densities of both seedling and adult M. serrulata were higher nearby to P. naegelii nests than in control areas. 4. Our findings show that P. naegelii switches its role from seed predator for most plant species to be the dominant seed disperser for M. serrulata, playing a key role in the distribution of adult plants.     

Ant behaviour and seed morphology: a missing link of myrmecochory

Seed dispersal by ants (myrmecochory) is mediated by the presence of a lipid-rich appendage (elaiosome) on the seed that induces a variety of ants to collect the diaspores. When seeds mature or fall onto the ground, these ant species transport them to their nest. After eating the elaiosome, the seed is discarded in nest galleries or outside, in the midden or farther away, where seeds can potentially germinate. The final location of seeds with their elaiosomes removed was evaluated to assess the importance of possible handles (structures that ants can grasp to carry) in transporting ants during re-dispersal experiments of seeds from nests of six species of ants. The results indicate that seeds remained within the nest because the ants were not able to transport them out of the nest. As a consequence of the elaiosome being removed, small ant species could not take Euphorbia characias seeds out of their nests. Only large ant species could remove E. characias seeds from their nests. Attaching an artificial handle to E. characias seeds allowed small ant species to redistribute the seeds from their nests. On the other hand, Rhamnus alaternus seeds that have a natural handle after the elaiosome removal were removed from the nests by both groups of ant species. If a seed has an element that acts as a handle, it will eventually get taken out of the nest. The ants’ size and their mandible gap can determine the outcome of the interaction (i.e. the pattern of the final seed shadow) and as a consequence, could influence the events that take place after the dispersal process.     

Odor of achlorophyllous plants’ seeds drives seed‐dispersing ants

Seed dispersal by ants is an important means of migration for plants. Many myrmecochorous plants have specialized appendages in their seeds called elaiosome, which provides nutritional rewards for ants, and enable effective seed dispersal. However, some nonmyrmecochorous seeds without elaiosomes are also dispersed by ant species, suggesting the additional mechanisms other than elaiosomes for seed dispersal by ants. The seeds of the achlorophyllous and myco‐heterotrophic herbaceous plant Monotropastrum humile are very small without elaiosomes; we investigated whether odor of the seeds could mediate seed dispersal by ants. We performed a bioassay using seeds of M. humile and the ant Nylanderia flavipes to demonstrate ant‐mediated seed dispersal. We also analyzed the volatile odors emitted from M. humile seeds and conducted bioassays using dummy seeds coated with seed volatiles. Although elaiosomes were absent from the M. humile seeds, the ants carried the seeds to their nests. They also carried the dummy seeds coated with the seed volatile mixture to the nest and left some dummy seeds inside the nest and discarded the rest of the dummy seeds outside the nest with a bias toward specific locations, which might be conducive to germination. We concluded that, in M. humile seeds, volatile odor mixtures were sufficient to induce seed‐carrying behavior by the ants even without elaiosomes.     

Seed predator deterrence by seed-carrying ants in a dyszoochorous plant, <Emphasis Type="Italic">Chamaesyce maculata</Emphasis> L. Small (Euphorbiaceae)

Seeds are often carried by omnivorous ants even if they do not carry elaiosomes. Although many seeds carried by ants are consumed, both seeds abandoned during the seed carrying and leftover seeds are consequently dispersed (dyszoochory). These non-myrmecochorous seeds do not necessarily attract ants quickly. Therefore, these seeds often seem to be exposed to the danger of consumption by pre-dispersal seed predators. We propose the hypotheses, “seed predator deterrence hypothesis” that plants may benefit from seed-carrying ants if they deter seed predators from visiting plants, and seed-carrying ants may play additional roles in plant reproductive success, besides dyszoochory by ants. To test the hypotheses, we investigated the abundance of seed-carrying ants of the species Tetramorium tsushimae Linnaeus and Pheidole noda Smith F., and of the seed predatory stinkbug, Nysius plebeius Distat, on the spotted sandmat, Chamaesyce maculata L. Small, of which the seeds have no elaiosomes but are consumed by both ants and bugs. In the field, ants and stinkbugs seldom encountered each other on the plant. The number of stinkbugs beneath the plants with ants was smaller than that beneath the plants without ants. In laboratory experiments, the number of stinkbugs on the shoot was smaller when ants were present than when they were absent. These results might support the seed predator deterrence hypothesis: the probability of seed predation by stinkbugs seems to be reduced by the ant visits on plants and/or the existence of ants beneath the plants. This study highlights a new ant–plant interaction in seed dispersal by ants.     

Seed handling behaviours of native and invasive seed‐dispersing ants differentially influence seedling emergence in an introduced plant

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DISPERSAL ECOLOGY OF CAREX PEDUNCULATA (CYPERACEAE), A NEW NORTH AMERICAN MYRMECOCHORE

Carex pedunculata is the first North American species of the Cyperaceae that is identified as a myrmecochore. Many morphological and phenological features of this species and its breeding system are interpreted as adaptive for seed dispersal by ants. In laboratory tests, workers of the ant species Aphaenogaster rudis carry the diaspores to the nest, eat the elaiosomes, carry larvae to the elaiosomes to feed, and deposit diaspores whose elaiosomes have been eaten with other nest debris. The achenes then germinate. Achenes will also germinate without any handling by ants. Workers will also transport diaspores with uneaten elaiosomes when the nest is disturbed. Greenhouse tests show that seedling growth is greatly inhibited if a diaspore remains near the parent plant and cohort seedlings. Field studies of natural populations identify rotting logs (the location of ant nests) as forest floor microsites for colonization of C. pedunculata and other myrmecochores. Ant nesting behavior may pattern much of the herb stratum. This species is self-compatible, and single seeds may start successful new populations. Three processes contribute to population growth: vegetative growth, germination of untransported diaspores, and germination of ant-transported diaspores.     

Unexpected relationships and valuable mistakes: non‐myrmecochorous Prosopis dispersed by messy leafcutting ants in harvesting their seeds

Abstract Ants generally disperse seeds while feeding on fruits or structures attached to the seed. Seed dispersal as a by‐product of seed predation (dyszoochory) was recognized in specialized harvester ants, but not in ants predating seeds opportunistically. Leafcutting ants are the main herbivores in much of the Neotropics, and they have been reported to remove fruits and seeds, but their role as seed predators and dispersers has not been acknowledged. Prosopis flexuosa D.C. (Fabaceae, Mimosoideae) is the most abundant tree species in the central Monte Desert, Argentina, and it is likely to depend on secondary animal dispersal. Mammalian frugivores are usually considered its main dispersers, but the opportunity for dispersal may be small since the removal of fruits and seeds by seed predators is very intense. The objective of this study was to identify which ant species interact with P. flexuosa fruits and to evaluate their relative importance as seed predators and dispersers. In a field experiment, whole and segmented pods were offered and several ant species exploiting the fruits were identified. Additionally, all pod segments remaining around nests of the three ant species able to remove them (the leafcutters Acromyrmex lobicornis Emery and Acromyrmex striatus Roger, and Pheidole bergi Mayr) were examined during and after the P. flexuosa primary dispersal season. Up to 753 pod segments and 90 sound seeds were found accumulated in a circle of 1 m radius over nests of A. lobicornis, and even more in an examined trail. Acromyrmex striatus left a smaller proportion of sound seeds and P. bergi left a smaller number of pod segments. All tendencies were similar during shorter known periods of accumulation. Leafcutting ants are acting as important seed predators, and ‘by mistake’ may be dispersing a key non‐myrmecochorous tree. This is an unexplored path in the seed dispersal cycle of P. flexuosa that challenges the tendency to predict interactions based on classifications made with other goals.     

Effects of ants,ground beetles and the seed-fall patterns on myrmecochory of Erythronium japonicum Decne. (Liliaceae)

Erythronium japonicum (Liliaceae) inhabits deciduous mesic forests of Hokkaido, northern Japan. Myrmecochory of this species was investigated, especially the dispersal frequency, the effect of seed predators and the seed fall pattern. In the quadrat census using marked seeds of E. japonicum, the ant Myrmica kotokui frequently transported the seeds. However, the frequency of seed removal was low and most seeds were dispersed as little as 1 m or less. The spatial distribution of E. japonicum individuals was nearly random and most seedlings were established 5–20 cm away from the fertile plants, indicating that even this small scale of seed dispersal contributes to avoiding crowding of seedlings. Some arthropods, e.g. springtails, spiders and ticks, hindered seed dispersal by devouring elaiosomes and seeds. Although ground beetle species also damaged seeds and elaiosomes, a few of them exhibited seed removal behaviour. E. japonicum dropped their seeds not all at once but bit by bit, taking 3–6 days to drop all seeds. This seed-fall pattern was effective in raising the frequency of seed removal by ants and reducing seed predation by some arthropods.     

Population dynamics of the shrub Acacia suaveolens (Sm.) Willd.: Dispersal and the dynamics of the soil seed-bank

Genet survival in seeds of Acacia suaveolens was examined through both dispersal and dormancy in the soil in populations near Sydney. Following initial passive seed-fall, the majority of seeds lie within a 1 m radius of the stem of the parent. Further dispersal is predominately mediated by ants. A. suaveolens seeds possess an elaiosome which attracts ants. When elaiosomes are removed, the potential for further dispersal of seeds is greatly reduced. Three species of ant disperse seeds of A. suaveolens and the fate of seeds following ant dispersal was observed to depend on the particular species of ant involved. Ants of both Iridomyrmex sp. and Pheidole sp. B are too small to drag seeds and, instead, ants of these species usually remove the elaiosome in situ, with little dispersal of the-seed resulting. Ants of Pheidole sp. A are larger and disperse seeds further, frequently taking them into their nests where the elaisosome is removed. Seeds are retained inside the nests and incorporated into the floors and walls of passageways and chambers. Several supposed ‘advantages’ of myrmecochory were examined but none were verified. Instead, two distinct ‘disadvantages’ were identified. These were: burial of seeds by ants of Pheidole sp. A into ‘unsafe sites’; and too deep a burial of seeds in nests for seeds to receive a stimulus to germinate during fires, and for seedlings to emerge successfully. Outside nests of Pheidole sp. A. seeds are concentrated in the top 5 cm of the soil, whilst within nests of these ants, seeds are found up to 15 cm deep. The dynamics of various components of the soil seed-bank were examined using seeds buried in nylon mesh containers. The seed-bank is persistent without annual recruitment to seedlings, enabling a population to persist as seeds after all above-ground plants have perished.     

Ant-seed interactions: combined effects of ant and plant species on seed removal patterns

Seed dispersal by ants (i.e. myrmecochory) is usually considered as a mutualism: ants feed on nutritive bodies, called elaiosomes, before rejecting and dispersing seeds in their nest surroundings. While mechanisms of plant dispersal in the field are well documented, the behaviour of the ant partner was rarely investigated in details. Here, we compared in laboratory conditions the foraging behaviour of two ant species, the omnivorous Lasius niger and the insectivorous Myrmica rubra to which seeds of two European myrmecochorous plants (Chelidonium majus and Viola odorata) were given. Ant colonies were simultaneously presented three types of items: entire seeds with elaiosome (SE), seeds without elaiosome (S) and detached elaiosomes (E). The presence of elaiosomes on seeds did not attract workers from a distance since ants first contact equally each type of items. Although ants are mass-recruiting species, we never observed any recruitment nor trail-laying behaviour towards seeds. For ants having contacted seed items, their antennation, manipulation and seed retrieval behaviour strongly varied depending on the species of each partner. Antennation behaviour, followed by a loss of contact, was the most frequent ant-seed interaction and can be considered as a “hesitation” clue. For both plant species, insectivorous Myrmica ants removed items in larger number and at higher speed than Lasius. This fits with the hypothesis of a convergence between odours of elaiosomes and insect preys. For both ant species, the small Chelidonium seeds were retrieved in higher proportion than Viola ones, confirming the hypothesis that ants prefer the higher elaiosome/diaspore-ratio. Thus, in these crossed experiments, the ant-plant pair Myrmica/Chelidonium was the most effective as ants removed quickly almost all items after a few antennations. The presence of an elaiosome body increased the seed removal by ants excepting for Myrmica which retrieved all Chelidonium seeds, even those deprived of their elaiosome. After 24 h, all the retrieved seeds were rejected out of the nest to the refuse piles. In at least half of these rejected items, the elaiosome was discarded by ants. Species-specific patterns and behavioural differences in the dynamics of myrmecochory are discussed at the light of ant ecology. Received 10 September 2007; revised 5 February 2008; accepted 5 March 2008.     

New mutualism for old: indirect disruption and direct facilitation of seed dispersal following Argentine ant invasion

The indirect effects of biological invasions on native communities are poorly understood. Disruption of native ant communities following invasion by the Argentine ant (Linepithema humile) is widely reported to lead indirectly to the near complete collapse of seed dispersal services. In coastal scrub in southeastern Australia, we examined seed dispersal and handling of two native and two invasive alien plant species at Argentine ant-invaded or -uninvaded sites. The Argentine ant virtually eliminates the native keystone disperser Rhytidoponera victoriae, but seed dispersal did not collapse following invasion. Indeed, Argentine ants directly accounted for 92% of all ant-seed interactions and sustained overall seed dispersal rates. Nevertheless, dispersal quantity and quality among seed species differed between Argentine ant-invaded and -uninvaded sites. Argentine ants removed significantly fewer native Acacia retinodes seeds, but significantly more small seeds of invasive Polygala myrtifolia than did native ants at uninvaded sites. They also handled significantly more large seeds of A. sophorae, but rarely moved them >5 cm, instead recruiting en masse, consuming elaiosomes piecemeal and burying seeds in situ. In contrast, Argentine ants transported and interred P. myrtifolia seeds in their shallow nests. Experiments with artificial diaspores that varied in diaspore and elaiosome masses, but kept seed morphology and elaiosome quality constant, showed that removal by L. humile depended on the interaction of seed size and percentage elaiosome reward. Small diaspores were frequently taken, independent of high or low elaiosome reward, but large artificial diaspores with high reward instead elicited mass recruitment by Argentine ants and were rarely moved. Thus, Argentine ants appear to favour some diaspore types and reject others based largely on diaspore size and percentage reward. Such variability in response indirectly reduces native seed dispersal and can directly facilitate the spread of an invasive alien shrub.     

SEED DISPERSAL OF TRILLIUM OVATUM (LILIACEAE) IN SECOND-GROWTH REDWOOD FORESTS

Trillium ovatum (Liliaceae) is myrmecochorous: its seeds bear large elaiosomes that are attractive to ants. Nevertheless, in coastal second-growth redwood forests of northern California, most seedlings occur in mixed-age clusters close to potential parents, suggesting that seed dispersal is limited. Ants were absent or rare at two relatively cool, moist study sites. At these sites, most seeds either eventually fell passively from fruits or were knocked to the ground by banana slugs that foraged on the elaiosomes. At two warmer, drier sites, a single species of ant, Lasius pallitarsis, dispersed the seeds but tended to remove the elaiosomes before returning to the nest. Thus at all sites a large number of seeds remained close to adults, accounting for the observed pattern of seedling distribution. The dispersal adaptations of T. ovatum and other redwood forest myrmecochores probably evolved in forests where seed-carrying ants were more common.     

Invasive Argentine ants (<Emphasis Type="Italic">Linepithema humile</Emphasis>) do not replace native ants as seed dispersers of <Emphasis Type="Italic">Dendromecon rigida</Emphasis> (Papaveraceae) in California,USA

We investigated the indirect effects of Argentine ant (Linepithema humile) invasions on patterns of seed dispersal and predation in the myrmecochorous tree poppy Dendromecon rigida in coastal San Diego County, California. Significantly more seeds were removed from ant-accessible seed stations at sites numerically dominated by a common harvester ant (Pogonomyrmex subnitidus), a native disperser of these seeds and a species sensitive to displacement by L. humile, than from those stations at sites where L. humile was in the majority. Predation of seeds was high, but variable, across sites, suggesting that reduced dispersal could result in increased seed predation in some habitats. Removal of elaiosomes did not affect the frequency with which predators removed seeds, but ants removed significantly more seeds with elaiosomes than without. In behavior trials, only P. subnitidus was able to carry seeds of Dendromecon rigida effectively. L. humile and a small native ant species, Dorymyrmex insanus, while displaying interest in the diaspores, were seldom able to carry whole seeds and, when they did, only carried them a few centimeters. Displacement of native harvester ants by L. humile appears to decrease the dispersal of Dendromecon rigida seeds and may be increasing loss of seeds due to predation.     

Signals Can Trump Rewards in Attracting Seed-Dispersing Ants

Both rewards and signals are important in mutualisms. In myrmecochory, or seed dispersal by ants, the benefits to plants are relatively well studied, but less is known about why ants pick up and move seeds. We examined seed dispersal by the ant Aphaenogaster rudis of four co-occurring species of plants, and tested whether morphology, chemical signaling, or the nutritional quality of fatty seed appendages called elaiosomes influenced dispersal rates. In removal trials, ants quickly collected diaspores (seeds plus elaiosomes) of Asarum canadense, Trillium grandiflorum, and Sanguinaria canadensis, but largely neglected those of T. erectum. This discrepancy was not explained by differences in the bulk cost-benefit ratio, as assessed by the ratio of seed to elaiosome mass. We also provisioned colonies with diaspores from one of these four plant species or no diaspores as a control. Colonies performed best when fed S. canadensis diaspores, worst when fed T. grandiflorum, and intermediately when fed A. canadense, T. erectum, or no diaspores. Thus, the nutritional rewards in elaiosomes affected colony performance, but did not completely predict seed removal. Instead, high levels of oleic acid in T. grandiflorum elaiosomes may explain why ants disperse these diaspores even though they reduce ant colony performance. We show for the first time that different elaiosome-bearing plants provide rewards of different quality to ant colonies, but also that ants appear unable to accurately assess reward quality when encountering seeds. Instead, we suggest that signals can trump rewards as attractants of ants to seeds.     

Effects of ant behaviour and presence of extrafloral nectaries on seed dispersal of the Neotropical myrmecochore Turnera ulmifolia L. (Turneraceae)

This paper describes the myrmecochory system of Turnera ulmifolia in a coastal sand dune matorral in Mexico. Turnera ulmifolia has elaiosome‐bearing seeds and extrafloral nectaries (EFNs). In ten quadrants (4 × 15 m) ant–seed interaction was monitored, and an interaction intensity index calculated and correlated with the number of seedlings. Seed removal rates by ants were surveyed every 2 h for 24 h, the ants being observed both on and beneath the plants. The role of the elaiosome in seed removal was evaluated by offering seeds with and without elaiosomes, and elaiosomes only. Finally, the effect of ant manipulation in seed germination was evaluated. There were 25 ant species associated with seeds and/or EFNs, the most frequently recorded being Monomorium cyaneum and Forelius analis. There was a positive correlation between the intensity index and seedling number per quadrant. There was significantly higher mean seed removal during the day than during the night (19.3% and 12.3%, respectively), and from beneath than on the plant (21.9% and 9.5%, respectively). The preference for elaiosomes only was also greater during the diurnal period, and when gathered on, rather than beneath, the plant. Seed manipulation by F. analis enhanced germination by T. ulmifolia. Seed removal, dispersal distances, seed predation and germination were largely determined by ant behaviour. The presence of EFNs may be influencing seed removal on the plant by attracting a specific assemblage of omnivorous ants. Among such assemblages associated with T. ulmifolia we encountered a variety of behaviours, with ant species either good at defending plants but bad at dispersing seeds, or vice versa. We discuss the way in which these two rewards, and the processes involved (defence and dispersion), could have interacted with each other and evolved. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 86 , 67–77.     

Pogonomyrmexcunicularius as the keystone disperser of elaiosome‐bearing Jatropha excisa seeds in semi‐arid Argentina

Myrmecochory or seed dispersal by ants is often described as a diffuse mutualism, because many of the ant species that function as partners are considered to be similar in terms of the frequency and consequences of their interactions. In this work, we test this assumption by conducting ant community surveys and seed removal experiments in six study sites located within a semi‐arid region of northwest Argentina. At each site, we characterized the ant assemblage that interacted with the seeds of Jatropha excisa Griseb. (Euphorbiaceae), an ant‐dispersed native shrub. Our results demonstrate that seed removal was dominated by one species, Pogonomyrmex cunicularius Mayr (Hymenoptera: Formicidae: Myrmicinae), which was responsible of 84% of the observed seed removal events. Although several ant species were attracted to the elaiosome‐bearing seeds of J. excisa, seed removal did not depend on ant community composition (species richness and ant activity) but was significantly influenced by the abundance of P. cunicularius. Its physical, behavioral, and ecological attributes are common with other ant species that have been characterized as keystone seed dispersers in other regions of the world. Nest feeding with marked seeds revealed that once P. cunicularius ants consume the elaiosomes, seeds are left inside the nests undamaged and at an appropriate depth for emergence. Our results support the hypothesis that myrmecochory is often an unevenly diffuse mutualism (i.e., one partner species is particularly important) and that at a local scale P. cunicularius is the keystone seed disperser of J. excisa.     

The evolution of ant-dispersal in a spring-ephemeral Corydalis ambigua (Papaveraceae): timing of seed-fall and effects of ants and ground beetles

Corydalis ambigua is one of major spring-ephemerals exhibiting ant-dispersal in Hokkaido, northern Japan Seed dispersal in this herb species was observed in a field To attract ants, C ambigua has small spherical seeds with large elaiosomes containing lipids In the quadrat census, 284 (11%) of 2560 seeds were moved away from their mother plants and 131 (46%) of them were recovered into ant nests The mean dispersal distance was 48 3 ± SD 34 4 cm Also the spatial distribution of C ambigua seedlings suggested that the seeds were frequently dispersed by ants The direct observations revealed that the main dispersal ant species were Myrmica kotokui and Lastus japomcus Additionally some ground beetles including Plerostichus spp often devoured elaiosomes and seeds Corydalis ambigua seeds were dropped for about two weeks in late May The collecting by pitfall trap revealed that, in this short period, the frequency of ant foraging on the ground was high and ground beetles were a little yet Moreover, in the seed removal experiments, seeds were more frequently removed in this seed-fall period while seed predation by ground beetles was intensive after the period     

The effect of limited visibility on vigilance behaviour and speed of predator detection: implications for the conservation of granivorous passerines

Ants have been traditionally considered either as predators or dispersers of seeds, but not both. That is, ant dispersal is restricted to myrmecochorous seeds, while almost all seeds removed by seed‐harvesting ants are eaten. However, harvesting ants might be simultaneously antagonistic and mutualistic towards seeds. This study analyzes the predation–dispersal relationship between seed‐harvesting ants and seeds of Lobularia maritima, a non‐myrmechorous perennial herb, in order to disentangle the dual role of ants as dispersers and predators of L. maritima seeds. The results obtained confirm the role of harvesting ants as both predators and dispersers of the non‐myrmechorous seeds of L. maritima. The removal activity of Messor bouvieri on L. maritima seeds is very important, particularly in autumn, which is the flowering and fruiting peak of this plant. It can be estimated that harvesting ants collect more than 85% of seeds, and almost 70% of them are effectively lost to predation. However, these granivorous ants also have drawbacks as seed dispersers. There is a relatively small percent of seeds collected by ants that escape predation, either because they are dropped on the way to the nest (16.4% of seeds harvested), or because they are mistakenly rejected on the refuse pile (0.9%). Abiotic dispersal of L. maritima seeds in the absence of ants occurs over very short distances from the plant stem. As seeds dispersed by ants reach a considerably greater distance than that obtained by gravity, this might represent a real advantage for the species, because it reduces intraspecific adult competition for seedlings, which directly influences seedling survivorship. These results challenge the generalization that seed removal by ants generally leads to successful seed dispersal if done by legitimate seed dispersers, or seed loss if done by seed consumers that eat them, and confirm that harvesting ants might have a dual role as both predators and dispersers of nonmyrmechorous seeds.     

A stochastic model of seed dispersal pattern to assess seed predation by ants in annual dry grasslands

A combined field experiment and modelling approach has been used to provide evidence that ants may be responsible for an observed lower patchiness and higher plant diversity in the neighbourhood of ant nests, within Mediterranean dry grasslands belonging to the phytosociological class Tuberarietea guttatae. The hypothesis was that seeds occurring in clumps may have a higher probability to be harvested than seeds having a scattered distribution. In order to test this hypothesis, four analysis steps were performed. First, pattern of seed production and dispersal of four species was recorded; two of them were more abundant next to ant nests (Tuberaria guttata, Euphorbia exigua), whereas the other two were more abundant away from ant nests (Bromus scoparius and Plantago bellardi). Second, a stochastic model was developed to simulate the observed dispersal patterns of each studied species. Third, 10 seed spatial arrangements in accordance to the distribution patterns created by the model were offered to ants and the location of predated seeds was recorded. Finally, the observed pattern of seed predation was matched to models performed by different distributions of probability. Results showed that the probability of being predated decreased as distance among seeds increased. This preference of ants for high concentration of food items holds down the dominant species sufficiently to allow the subordinates to survive, thus increasing diversity near nests. The observed higher frequency of small-seeded, small-sized, or creeping therophytes close to the ant nests can be therefore seen as an example of indirect myrmecophily.     

The possible advantage of myrmecochory in diplochorous species: A test on two Mediterranean Euphorbia species

Myrmecochory commonly complements the advantages of ballistic dispersal in diplochorous species. We studied the role of the elaiosome in two populations of the two diplochorous Mediterranean spurges Euphorbia boetica and E. nicaeensis, which share an efficient ballistic dispersal mechanism followed by secondary removal by ants. They differ in elaiosome persistence, as most E. boetica seeds lose the elaiosome during explosive dispersal. Self-assessed dietary preferences with seeds with and without elaiosomes of each species showed differences in behaviour among and within ant species. In general, the absence of elaiosome entailed a decrease in the number of disperser ant species interacting with the seeds, whereas the number of predatory ants remains invariable. However, in one population of E. nicaeensis, experimental elimination of the elaiosome did not affect seed removal by mutualistic ants. On the other hand, analysis of refuse piles of the granivorous Messor marocanus and M. bouvieri suggests that they act as seed predators in E. boetica, whereas unintentional dispersal can be important in E. nicaeensis. We suggest, therefore, that the presence of the elaiosome in the seeds of the studied spurges increases the interaction with disperser ant species, but the possible dispersal advantage is not apparent and is spatially variable.